The efficient and reliable operation of organic optoelectronic devices, such as organic light-emitting diodes (OLEDs) and solar cells, requires facile and balanced electron and hole transport between organic layers and electrodes. Large offsets between the work functions of the electrodes and energy levels of organic materials would cause high energy barriers for charge injection or acceptance, leading to high operation voltages, low quantum efficiency, and fast device degradation. State-of-the-art OLEDs suffer more from poor hole injection than electron injection. This is because the commonly-used hole-injecting electrode, indium-tin-oxide (ITO), has a work function about 4.7 eV, which is about 1 eV lower than the highest occupied molecular orbital (HOMO) of typical hole transport layer (HTL) and host materials. Considerable efforts have been devoted to the modification of the ITO/HTL interface for the enhancement of hole injection. The following three types of strategies are often adopted. (i) A thin organic interlayer, such as copper phthalocyanine (CuPc) and Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS), is introduced between the ITO anode and HTL as a hole injection layer (HIL). The HIL has a HOMO between the Fermi level of ITO and HOMO of the HTL. Its addition thus creates a ladder-like energy structure that facilitates hole injection. (ii) A nanometer-thick inorganic insulator (mostly a metal oxide like MoO3 and Pr2O3) is deposited on ITO to sustain a positive voltage drop, which effectively displaces the Fermi level of ITO downward. It can thus reduce the hole injection barrier. (iii) Surface treatments by plasma, ultraviolet ozone, and wet chemicals may be performed to modify the surface chemical states or introduce monolayer dipoles pointing toward the organic structure. In many cases, a simple treatment leads to an increase in the work function of ITO, and thus a reduced interfacial potential barrier.
Among different strategies, surface treatment is considered as the most convenient one because it can be easily incorporated into the OLED fabrication without adding structural complexity. Pretreatment of ITO with O2 plasma is now widely used, often in conjunction with the addition of a HIL to improve hole injection in OLEDs. O2 plasma not only effectively removes surface contamination, but also induces a Sn—O dipole layer, raising the work function of ITO by as much as 0.5 eV. However, the work function of O2 plasma-treated ITO is still considerably lower than what is required for facile hole transport, particularly in short-wavelength OLEDs. Recently, Helander et al. developed a new treatment method which exposed ITO to o-dichlorobenzene vapor under UV radiation. The treatment raised the ITO work function from 4.7 eV to 6.1 eV, and enabled direct injection of holes into the light-emitting layer of a simplified green OLED. This approach, however, is not very compatible with OLED fabrication process. Furthermore, the treatment only creates relatively weak In—Cl bonds, which appear to be unstable. Organic solar cells fabricated on o-dichlorobenzene-treated ITO/glass were found to suffer rapid performance degradation.